Aerogels are chemically inert, highly porous ceramic materials. Generally, these materials are produced by forming a gel containing a solvent and a porous solid component, and then removing the solvent to leave behind the porous solid. Removal of the solvent while preserving the porous solid structure is difficult because the gel often shrinks upon removal of the solvent and causes the porous solid structure to crack and break. This obstacle has been overcome by transforming the solvent within the gel into a vapor above its supercritical point, and allowing the vapor to escape and leave the intact porous solid structure. The first true aerogels were produced by exchanging water in the gel with alcohol that was then converted to a supercritical fluid and allowed to escape. This produced an aerogel that was transparent, low density, and highly porous. A major advance took place when this technique was combined with the application of sol-gel chemistry to prepare silica aerogels. This process replaced the sodium silicate that was typically used with an alkoxysilane, (for example, tetramethyorthosilicate, TMOS). Hydrolyzing TMOS in a solution of methanol produced a gel in one step that was called an “alcogel”. This eliminated two of the drawbacks of the previously used procedure, namely, the water-to-alcohol exchange step and the presence of inorganic salts in the gel. Drying these alcogels under supercritical alcohol conditions produced high-quality silica aerogels. In subsequent years, this approach was extended to prepare aerogels from a wide variety of metal oxide aerogels.
Recent advances have produced aerogels that are the product of a sol-gel process, whose final stage involves extracting the pore-filling solvent with liquid CO2. The latter is gasified supercritically and is vented off, leaving behind a very low density solid (0.002-0.8 gram cm−3), with the same volume as the original hydrogel and a chemical composition identical to glass.
Aerogels have been considered for thermal insulation, catalyst supports, or as hosts for a variety of functional materials for chemical, electronic, and optical applications. However, practical application of aerogels has been slow because aerogels are brittle and hygroscopic. These properties cause several known aerogels to absorb moisture from the environment which causes them to collapse due to the capillary forces developing in the pores. Therefore, aerogels having superior strength characteristics that would overcome the deficiencies of the past would be useful in a large variety of applications.